The invention relates to an optical arrangement for immersion lithography, having at least one component to which a hydrophobic coating is applied, the hydrophobic coating being exposed to UV radiation during operation of a projection lens, the at least one component being wetted at least in part by an immersion fluid during operation of the projection lens. The invention further relates to a projection exposure apparatus including such an optical arrangement.
As a rule, wetting of optical components with liquids, in particular with water, has a negative effect on their optical characteristics. Wetting can, for example, result in the formation of contaminating substances such as salts on their surfaces. In order to prevent wetting of optical elements with water, or in order to quickly remove water from said optical elements, it has been known to provide optical elements with hydrophobic coatings. In the context of this application the term “hydrophobic coating”, as is generally the case, refers to a coating whose surface encloses a contact angle of 90° or above with water.
JP 2003-161806 A describes an optical element with an anti-reflection coating in which a water-repellent layer is formed in a non-coated region of the glass substrate. In this arrangement the water-repellent coating can be formed adjacent to the anti-reflection coating or on a lateral edge of the glass substrate. In this way any lodging of humidity in the space between the optical element and an associated holding structure is to be prevented.
U.S. Pat. No. 5,494,712 describes a method for applying a layer of a polymer to a substrate to reduce its wetting by water. The layer preferably includes one or several organo-silicon compounds, e.g. silanes or siloxanes, and is applied using plasma enhanced chemical vapour deposition (PECVD).
The issue of wetting is significant also in microlithography, in particular in immersion lithography. In microlithography, for the purpose of producing semiconductor components, structures on a mask are imaged with a projection lens at a reduced scale onto a light-sensitive substrate. In order to achieve the high resolution required for such applications, illumination radiation in the UV wavelength range, typically below 250 nm is used. In order to achieve a further increase in resolution and depth of field, in immersion lithography, an immersion fluid, usually distilled water, is arranged between the last optical element of the projection lens and the light-sensitive substrate so as to increase the refractive index. In this arrangement the last optical element of the projection lens is at least partly wetted by water, so that in immersion lithography water-repellent and water-resistant coatings are used with increased frequency.
From JP 2005-268759 A an optical component has become known, which is arranged in a projection exposure apparatus for immersion lithography and at least on one surface is exposed to the illumination radiation. The surface has a bonding layer made of silicon dioxide (SiO2), magnesium fluoride (MgF2) or calcium fluoride (CaF2), onto which a water-repellent layer including an amorphous fluoropolymer has been applied.
JP11-149812 describes an optical element to which a water-repellent protective layer, preferably including a fluorocarbon polymer, has been applied to a reflection-increasing or a reflection-reducing multilayer system, so as to provide protection against the ingress of humidity from the ambient air. The thickness of the protective layer is between 1 nm and 10 nm so as to prevent excessive absorption of radiation by the fluorocarbon polymer at wavelengths of less than 250 nm.
EP 0895113 A2 describes an assembly having a holding device and a component pasted up with an adhesive, which component transmits radiation in the ultraviolet spectral range. The adhesive is curable by UV light, and between the transparent component and the adhesive, in the region of the adhesive, a thin layer is applied which transmits light in a spectral range suitable for curing the adhesive, and which reflects or absorbs to a high degree UV light from a useful spectral range within the spectral range transmitted by the transparent component, as a result of which the adhesive is protected against UV radiation in this wavelength range.
From printed publication US 2006/0240365 a method for protecting a water-sensitive lens element has become known. In this method, at an edge region of the lens surface a thin UV-resistant layer that absorbs UV radiation is generated, which layer consists of a metal oxide. A further protective layer is applied to the metal oxide layer, which further layer can, for example, be a polyurethane layer. The two layers are intended to protect the lens element, which can, for example, consist of water-sensitive calcium fluoride, from being dissolved by an immersion fluid.
However, hydrophobic coatings can not only be used, as described above, for the purpose of preventing the wetting of optical surfaces. Instead, hydrophobic coatings can also be applied to a location on a component, at which location wetting by water is inevitable, e.g. in order to produce an essentially meniscal water column. Applying such a coating can be advantageous, e.g. in the case of interferometric measuring of a projection lens for microlithography, with respect to its optical imaging properties and to any imaging errors, prior to the use of said lens. For such measuring, the projection lens is arranged above a corresponding measuring device, and an immersion fluid is placed between the measuring device and the projection lens so that measuring can take place at conditions experienced in practical application. In most cases super-pure water is used as an immersion fluid, with a ring around the optical components of the measuring device preventing said super-pure water from running off. This ring must have a hydrophobic surface in order to produce a convex and thus upwardly curved water meniscus. It is important that impeccable contact to the last optical component of the lens, which component faces the measuring device, is reliably ensured at all times.
A general problem in the use of hydrophobic coatings in immersion systems consists of the possibility of said coatings being damaged or destroyed by the UV radiation that during operation of the projection lens impinges on said coatings. Furthermore, as a result of UV radiation the hydrophobic characteristics of the coating may degrade so that in extreme cases said coating can develop hydrophilic characteristics. In particular, practical applications have shown that the hitherto-used materials in the construction of the ring with the hydrophobic surface have already changed after a short time, as far as their surface characteristics are concerned, to the effect that with increased duration of radiation there was a reduction in their hydrophobic characteristics. This relates in particular to radiation with lasers which use increasingly shorter wavelengths, e.g. at a wavelength of 193 nm and shorter. This means that already after a brief period of operation it was no longer possible to provide a stable convex water meniscus, and as a result of this the immersion column between the lens and the measuring device broke, with measuring having to be interrupted.